Such downhole seals are of a balloon type and the inflating medium is liquid. Downhole seals of this kind have been used for a number of years.
In inflated condition they serve as plugs in the well bore, and in one application case they may prevent the passing of undesired fluid, e.g. water, which is produced in a zone of the formation beneath an oil and/or gas-producing formation zone. In such a case a downhole seal will be placed immediately above the upper boundary layer of the water-producing formation zone.
In another application case two spaced-apart inflatable downhole seals are used, and here too the seals prevent undesired fluid (water) from passing, but in this case it is sought to exploit oil and/or gas of a formation beneath the water-producing zone. In such a case there may also exist an oil-producing formation zone above the water-producing zone.
In the latter case, a pipe connection is established leading through the lower inflated seal, through the well bore section between the seals, in which water is being produced, and through the upper inflated seal. Oil/gas in the formation zone beneath the water-producing zone is thus transported through the lower seal, then through the pipe connecting the seals and eventually through and out of the upper seal, this oil/gas not having been in contact with the water in the zone located between the seals.
In the following description the invention will be explained in connection with vertical well bores, and the terms "above" and "below" will then be easily understood. In deviated wells and horizontal wells "above" means "closer to the well head" and "below" means "further from the well head". Of course, inflatable balloon type seals according to the invention are not limited to application in well bores of a particular orientation, but may be used in vertical well bores, deviated wells, extended reach wells and horizontal wells.
When the seal(s) is (are) in the inflated condition in a well bore with its (their) circumferential portion bearing in a tightening manner against the formation wall, temperature changes in the environment immediately adjacent the well bore will give rise to internal pressure changes in the inflated seals. By high temperature developing in the well bore area, in which the inflated seal forms a plug, it happens that the seal bursts in the same way as an inflated balloon. By low temperature, the bearing and tightening pressure of the seal on the wall of the well bore is reduced, and it therefore happens that the seal loosens.
It is known to precondition wells before such seals are set, so that the seals may be set at a temperature which is considered to be normal operating temperature. Preconditioning of a well is done by regulating the flow rate (production rate or injection rate) of the well and is time consuming, expensive and uncertain as to the result.
Likewise, it is known to fill the seal with curable materials--cement mixture or liquid, subsequently setting epoxy resin composition--instead of a liquid inflating medium which remains in its liquid form. A disadvantage of a hard, firm plug-like seal is that it is not that easily removed. Indeed, it may be drilled to pieces, but that is an expensive operation.